Dual frequency ultrasound transducer

ABSTRACT

A dual frequency ultrasound transducer includes a high frequency (HF) transducer and a low frequency (LF) transducer that is positioned behind the high frequency transducer. An intermediate layer is positioned between the low frequency transducer and the high frequency transducer to absorb high frequency ultrasound signals. An alignment feature on the low frequency transducer is positioned with respect to a fiducial that is marked at a known position with respect to high frequency transducer elements of the HF transducer to align low frequency transducer elements of the LF transducer with the HF transducer elements.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation of U.S. patent applicationSer. No. 16/051,060 filed Jul. 31, 2018, now allowed, which is relatedto, and claims the benefit of, U.S. Provisional Patent Application No.62/666,519 filed May 3, 2018, which is herein incorporated by referencein its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The subject of the preset application was made with government supportunder grant number 5100220, awarded by the National Institutes of Health(NIH)—Federal Reporting and Grant Number RO1CA189479. The Government hascertain rights in the invention.

TECHNICAL FIELD

The disclosed technology relates to ultrasound transducers and inparticular to dual frequency ultrasound transducers.

BACKGROUND

In conventional ultrasound imaging, ultrasonic acoustic signals aredirected towards a region of interest and the corresponding reflectedecho signals are detected. Characteristics of the echo signals such astheir amplitude, phase shift, Doppler shift, power etc. are analyzed andquantified into pixel data that are used to create an image of orrepresentation of flow. With conventional single transducer ultrasoundimaging, the received ultrasound echo signals are in the same frequencyrange as the transmitted ultrasound signals.

Another approach to performing ultrasound imaging is to apply ultrasonicacoustic signals to the region of interest at one frequency and tocapture and analyze the received echo signals at another frequency suchas at one or more harmonics of the transmitted ultrasound signals.Typically, the harmonics have a frequency that is 3-5 times that of thetransmitted signals. One specific use of harmonic imaging is imagingtissue with contrast agents. Contrast agents are generally fluid orlipid encapsulated microbubbles that are sized to resonate at aparticular transmitted ultrasound frequency. Exposure to ultrasound inthe body at the resonant frequency of the microbubbles, causes thebubbles to rupture and produce non-linear ultrasound echoes having amuch higher frequency than the applied ultrasound. For example,non-linear microbubbles can be designed to resonate at 1-6 MHz butproduce echo signals in the range of 10-30 MHz. The high frequency echosignals allow detailed images of tissue structures to be produced andstudied such as the micro-vasculature surrounding tumors in clinical orpre-clinical settings.

The most conventional way of performing dual frequency imaging is with amechanically scanned, single element transducer having confocal low andhigh frequency transducer elements. While such transducers work well,faster scanning can be performed with transducer arrays that can beelectronically steered. Such transducers generally have a low frequencytransducer array and a high frequency transducer array that are alignedwith each other. One problem with dual frequency transducers is aligningthe low and high frequency arrays. In a 30 MHz high frequency phasedarray, the element size (e.g. ½λ or less) is about 25 microns. At 50MHz, the element size is approximately 15 microns. For comparison, atypical human hair is approximately 80 microns in diameter. Theprocedure required to align the arrays often requires makingmicro-adjustments to the position of the high and low frequencytransducers on a wet bench and then adhering them together when the bestmatch is found. This is both time consuming and costly. The technologydiscussed herein relates to an improved dual frequency transducer thatis easier and/or less costly to manufacture.

SUMMARY

A dual frequency ultrasound transducer includes a high frequencytransducer and a low frequency transducer that is positioned behind thehigh frequency transducer. An alignment feature on the low frequencytransducer is positioned with respect to a fiducial that is marked at aknown positioned with respect to the high frequency transducer. In oneembodiment, one or more fiducials are marked on an intermediate layerthat is positioned between the high frequency transducer and the lowfrequency transducer. The intermediate layer is configured to absorbhigh frequency ultrasound signals while passing lower frequencyultrasound signals.

In one embodiment, the high frequency transducer includes a supportframe that includes an alignment post that fits with a cooperating slotor keyway on the intermediate layer to align the intermediate layer withthe support frame and the high frequency transducer elements. One ormore fiducials on the intermediate layer are positioned at locationsreferenced to the high frequency transducer elements to align the lowfrequency transducer to the high frequency transducer elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of a piezoelectric layer of a high frequency (HF)transducer in accordance with some embodiments of the disclosedtechnology;

FIG. 1B is a top view of a piezoelectric layer of a low frequency (LF)transducer in accordance with some embodiments of the disclosedtechnology;

FIG. 2 is a partial isometric view of a conductive frame andpiezoelectric layer of a high frequency transducer array in accordancewith some embodiments of the disclosed technology;

FIG. 3 is an isometric view of a piezoelectric layer, a flex circuit anda conductive frame with alignment features formed on a rear side of theframe in accordance with some embodiments of the disclosed technology;

FIG. 4 is an isometric view of an intermediate layer to be positionedbetween the low frequency transducer and the high frequency transducerin accordance with some embodiments of the disclosed technology;

FIG. 5 is a cross-sectional view of a dual frequency transducer with theintermediate layer in place over a high frequency transducer;

FIGS. 6A-6D are top, side and end views of an intermediate layer alignedwith a high frequency transducer in accordance with an embodiment of thedisclosed technology;

FIG. 7A is an isometric view of piezoelectric layer of a low frequencytransducer in accordance with some embodiments of the disclosedtechnology;

FIG. 7B shows one embodiment of a low frequency transducer elementdicing pattern in accordance with an embodiment of the disclosedtechnology;

FIG. 8 is an isometric view of a low frequency transducer in accordancewith some embodiments of the disclosed technology;

FIGS. 9A-9C are end, top and side views of a low frequency transducer inaccordance with some embodiments of the disclosed technology;

FIGS. 10A and 10B show how connections are made between ground andsignal traces in a flex circuit to the electrodes for the low frequencytransducer elements and common transducer ground electrode in accordancewith one embodiment of the disclosed technology;

FIG. 11 is a cross-sectional view of a dual frequency transducer showingan alignment of high and low frequency transducer elements in accordancewith some embodiments of the disclosed technology;

FIG. 12 shows a housing for a dual frequency transducer in accordancewith an embodiment of the disclosed technology; and

FIGS. 13A-13H show a number of beam plots from a low frequencytransducer in accordance with an embodiment of the disclosed technology.

DETAILED DESCRIPTION

The technology shown in the figures and described below relates to adual frequency ultrasound transducer. Components of a dual frequencytransducer are shown for the purpose of explaining how to make and usethe disclosed technology. It will be appreciated that the illustrationsare not necessarily drawn to scale.

FIG. 1 is a top view of a piezoelectric layer 50 for a high frequencyultrasound transducer in accordance with one embodiment of the disclosedtechnology. In one embodiment, the layer 50 has an outer frame 52 thatsurrounds a sheet 54 of PZT, single crystal piezoelectric or other knownpiezoelectric material. In one embodiment, the frame 52 is apre-machined, non-conductive alumina ceramic with a center cut out areathat is slightly larger than the outer dimensions of the sheet 54 ofpiezoelectric material. In other embodiments, the frame 52 can be madeof a conductive material such as molybdenum or graphite. The frame 52preferably has a coefficient of thermal expansion that is similar tothat of the piezoelectric material so that the piezoelectric materialdoesn't crack during manufacture, handling or use. An adhesive 56 suchas EPO-TEK 301 epoxy fills a gap between the frame and the piezoelectricmaterial. The piezoelectric material 54 is diced with a patterning laseror a dicing saw to create a number (e.g. 128, 256 or fewer or a largernumber) of piezoelectric elements (not separately shown). By surroundingthe sheet 54 of piezoelectric material with the frame 52, the kerf cutsthat define the individual transducer elements can extend across theentire width of the sheet of piezoelectric material.

If the frame 52 is made of a non-conductive material, a cut-out area ornotch 60 on one or both sides of the frame is filled with a conductiveepoxy or can include a number of conductive vias to provide anelectrical path from a front surface of the frame to a rear surface ofthe frame. More details about the construction of an ultrasoundtransducer with a surrounding frame can be found in commonly-owned U.S.patent application Ser. No. 15/993,156 filed May 30, 2018 and U.S.Provisional Application No. 62/612,169 filed Dec. 29, 2017, which areherein incorporated by reference in their entireties.

FIG. 1B shows a piezoelectric layer 70 of a low frequency ultrasoundtransducer that in one embodiment, is constructed in the same manner asthe piezoelectric layer of the high frequency transducer shown in FIG.1A. An alumina frame 72 surrounds a sheet 74 of piezoelectric material.The frame 72 has a central opening that is sized to form a gap betweenthe edges of the opening and the edges of the piezoelectric sheet 74.The gap is filled with a non-conductive epoxy 76. The low frequencyarray has fewer transducer elements than the high frequency transducer.In one embodiment, the low frequency transducer has 32 individuallyaddressable transducer elements but could have fewer or more elements. Apair of cut-out areas or notches 80 on either side of the frame 72 canbe filled with a conductive epoxy or can include conductive vias throughthe frame to provide an electrical path from a front surface of the lowfrequency array to the rear surface of the frame.

FIG. 2 is an isometric view of a conductive support frame 100 that is tobe secured to a rear surface of the piezoelectric layer 50 of the highfrequency transducer. The frame 100 supports one or more flex circuits(not shown) having signal traces therein that are electrically connectedto corresponding electrodes on each of the high frequency transducerelements. In addition, the support frame 100 provides electromagneticshielding for the rear surface of the transducer elements. A commonground electrode on the front surface of the piezoelectric layer 50 isconnected to a ground plane in the flex circuits via a conductive paththat includes the conductive support frame 100. In one embodiment, thesupport frame 100 is secured to the piezoelectric layer 50 with aconductive epoxy. Also shown in FIG. 2 are one or more matching layersand a lens that are positioned ahead of (e.g. in front of) thepiezoelectric layer 50.

FIG. 3 shows one flex circuit 104 attached to the support frame 100. Insome embodiments, the high frequency transducer includes two flexcircuits with the signal traces of one flex circuit electrically coupledto the even numbered transducer elements and the signal traces of theother flex circuit electrically connected to the odd numbered transducerelements. Electrical connections are made from the individual transducerelements on the rear surface of the piezoelectric layer to the metalsignal traces in the flex circuits. Connections between the transducerelements and the metal signal traces in the flex circuits can be madeusing the techniques described in U.S. Patent Publication No.2017/0144192 or U.S. Pat. No. 8,316,518, which are herein incorporatedby reference in their entireties. After the connections are made, anopen back side of the support frame is filled with an epoxy material 110such as EPO-TEK 301. In one embodiment, the height of the epoxy 110 inthe frame extends above the rim of the support frame 100. As shown inFIG. 3, the epoxy 110 is molded or laser machined to form a pair ofalignment posts 112 a, 112 b and a recess area 114 that is behind thehigh frequency transducer elements. In one embodiment, the depth of therecess area 114 is about 4.2 mm. The alignment posts 112 a, 112 b areconfigured to align an intermediate layer over the high frequencytransducer as will be described below. A pair of fiducials 120 a, 120 bare marked with a laser on each alignment post at a known distance fromthe high frequency transducer elements. The fiducials 120 a, 120 b serveas a reference so that other components of the dual frequency transducercan be placed at known positions with respect to the high frequencytransducer elements.

FIG. 4 illustrates one embodiment of an intermediate layer 150 that isplaced behind the high frequency transducer. In one embodiment, theintermediate layer 150 is made of silicone powder-doped EPO-TEK 301epoxy. The size of the silicone powder is in the nanometer range suchthat it highly attenuates high frequency (HF) ultrasound and passes lowfrequency (LF) ultrasound. The intermediate layer is sized to absorbhigh frequency ultrasound to a specified level, such as 65 dB, but topass low frequency signals transmitted from, or to be received by, thelow frequency transducer. The top of the intermediate layer 150 includesa pair of slots or keyways 152 a, 152 b that cooperate with thealignment posts 112 a, 112 b on the back side of the frame 100 to alignthe intermediate layer 150 at a known position with respect totransducer elements of the high frequency transducer. In one embodiment,the intermediate layer 150 is secured to the frame 100 using the sameepoxy that fills the back side of the frame to avoid acousticdiscontinuities at the bond line.

Once the intermediate layer is secured to the frame 100, additionalfiducials 154 a, 154 b can be placed on the intermediate layer asmeasured from the fiducials 120 a, 120 b that are marked on the supportframe (i.e. on the filler epoxy at the back side of the support frame).

FIG. 5 shows a cross section of the high frequency transducer with theintermediate layer 150 secured to the back of the support frame 100. Asshown, the high frequency transducer comprises a lens 130 designed tofocus the high frequency ultrasound signals at a specific depth. Typicallens materials include polymethylpentene (tradename TPX), cross-linkedpolystyrene (tradename Rexolite) or polybenzimidazone (tradenameCelezole). However, other non-attenuating plastic materials could alsobe used. Behind the lens are one or more quarter wave matching layers132 that match the impedance of the piezoelectric layer 50 to theimpedance of the lens 130. The one or more matching layers are generallyformed by adding particles to an epoxy to adjust its acoustic impedance.A metallic (e.g. gold or gold+chromium) common ground electrode isdeposited on the front surface of the piezoelectric layer 50 iselectrically coupled to the conductive support frame 100 through theslots or vias in the frame 52 surrounding the PZT material. An appliedmetal coating on the rear or proximal surface of the piezoelectric layeris formed into conductive paths from individual transducer elements tothe signal traces in the flex circuit. The layer of epoxy 110 covers theproximal side of the transducer elements and fills in the rear side ofthe support frame. In one embodiment, the recess 114 formed in the epoxybehind the high frequency transducer array is sized such that theintermediate layer is larger in the elevation and azimuthal dimensionsof the transducer than the size of the transducer elements so that theintermediate layer extends over the ends of high frequency transducerelements.

FIGS. 6A-6D illustrate a pair fiducials 154 a, 154 b that are marked ona top surface of the intermediate layer 150. The fiducials 154 a, 154 bare measured with respect the fiducials 112 a, 112 b that are marked onthe epoxy in the support frame. The fiducials 154 a, 154 b are marked onthe intermediate layer once the intermediate layer is secured to thesupport frame behind the high frequency transducer. In the embodimentshown, the fiducials 112 a, 112 b, 154 a, 154 b are crosses made with alaser.

FIG. 7A shows an embodiment of a piezoelectric layer 70 for the lowfrequency transducer. In the embodiment shown, the piezoelectric layer70 also includes an alumina frame surrounding a sheet of PZT or otherpiezoelectric material. In one embodiment, the sheet of PZT is dicedinto a number of columns of triangular pillars as a 1-3 composite asbest shown in FIG. 7B. In the example shown, each column 72 a, 72 b, 72c of triangular elements 74 is treated as a single transducer element.In FIG. 7B, only 12 columns of transducer elements are shown forpurposes of illustration. In one embodiment, the low frequencytransducer includes 32 columns of triangular elements. However, the lowfrequency transducer could include fewer or a greater number of columnsas desired. Furthermore, each transducer element in the low frequencytransducer may have other shapes such a small squares orrectangular-shaped elements.

FIG. 8 is an isometric view of a low frequency transducer in accordancewith an embodiment of the disclosed technology. The low frequencytransducer includes the piezoelectric layer 70, one or more matchinglayers 160 positioned ahead of (e.g. in front of) the piezoelectriclayer 70 and a backing layer 170 positioned behind the transducerelements. A flex circuit 180 includes signal traces that areelectrically connected to individual low frequency transducer elements.The low frequency transducer also includes a pair of alignment tabs 162a, 162 b. In one embodiment, the alignment tabs are Kapton™ sheetsinserted into an epoxy matching layer at a position away from theposition of the low frequency transducer elements so as not to interferewith the operation of the low frequency transducer. Other materials forthe alignment tabs could also be used such as alumina. Holes 164 a, 164b are placed in the alignment tabs at a known distance from the lowfrequency transducer elements. The holes are placed over a fiducial onthe high frequency transducer in order to precisely align the lowfrequency transducer elements with respect to the high frequencytransducer elements.

In one embodiment, the holes 164 a, 164 b in the alignment tabs 162 a,162 b are placed over the fiducials on the intermediate layer. If thealignment tabs are long enough, the holes in the alignment tabs could beplaced over the fiducials marked on the support frame in order to alignthe low frequency transducer with respect to the high frequencytransducer.

FIG. 9A is a cross-sectional view of a low frequency transducer inaccordance with one embodiment of the disclosed technology. Thetransducer includes the piezoelectric layer 70 and two matching layers160 positioned in front of the piezoelectric layer. The matching layersmatch the impedance of the low frequency piezoelectric elements to theimpedance of the intermediate layer to which the low frequencytransducer is secured. A backing layer 170 is positioned behind the lowfrequency transducer elements. As shown in FIG. 9C, the alignment tabs162 a, 162 b are part of a matching layer ahead of the low frequencytransducer elements. The holes 164 a, 164 b in the alignment tabs arepositioned at a known distance from the low transducer elements as canbe seen in FIG. 9B. A flex circuit 180 extends out from under thebacking layer where connections are made to the transducer elements.

FIGS. 10A and 10B show one way of connecting the signal and groundtraces of a flex circuit to the corresponding electrodes in the lowfrequency array. As shown in FIG. 10A, the flex circuit 180 includes anumber of signal traces 182 and a ground trace 184. The ground trace 184is electrically connected to the ground electrode on the front of thelow frequency array with a conductive epoxy 186 joined to conductiveepoxy 188 in the notches 80 in the frame 70 or to vias in the frame. Asshown in FIG. 10B, the exposed ends of the signal traces 182 can beadhesively secured to the transducer elements with a conductive epoxy.Because the transducer elements of the low frequency transducer arerelatively large, they can be easily aligned signal traces 182 in theflex circuit 180.

FIG. 11 is a cross sectional view of the dual frequency transducer inaccordance with one embodiment of the disclosed technology. The lowfrequency transducer elements 71 are directly aligned with the highfrequency transducer elements 51 by placing the holes on the alignmenttabs 162 a, 162 b over the fiducials on the intermediate layer. Theholes in the alignment tabs are placed at a known location with respectto the low frequency transducer elements and the fiducials are preciselyplaced at a known location with respect to the high frequency transducerelements. Therefore, placing the holes over the fiducials on theintermediate layer precisely positions the low frequency transducerelements with respect to the high frequency transducer elements withouthaving to align the transducer on a wet bench.

FIG. 12 shows one embodiment of a transducer housing 200 surrounding thedual frequency transducer array. In the embodiment shown the housingincludes separate cables 202, 204 connecting the low and high frequencytransducers. In one embodiment, the low frequency transducer can be usedalone to image tissue with low frequency ultrasound (e.g. in a frequencyrange of 4-10 MHz). Similarly, the high frequency transducer can be usedto image tissue with high frequency ultrasound (e.g. in a frequencyrange of 20-50 MHz). Alternatively, both transducers can be used toperform imaging such as harmonic or contrast agent imaging where aregion of interest is insonified with signals from the low frequencytransducer and ultrasound echo signals at harmonics of the excitationfrequency are detected with the high frequency transducer.

FIGS. 13A-13H are some sample beam plots of a low frequency transduceroperating in a plane wave imaging mode. In one embodiment, a 32-elementlow frequency transducer is positioned behind a 256 element highfrequency transducer. Electrical matching using 1:3 mini-transformersbring the impedance of the low frequency transducer elements from about500 ohms to about 47.2+/−2.6 ohms with a phase of minus 60.1+/−5.5degrees. The single low frequency element peak-peak pressures measuredat 1.7 mm. from the lens were increased from 0.77 kPaN without matchingto 1.51 kPaN with matching in the 6-140V range with a variation of about+5% across the elements. In plane wave imaging, pressure is 3 timeshigher than for a single element. Focusing is observed in the elevationplane at 5.5 mm with a 6 dB beamwidth of 2.7 mm (FIGS. 13A,13C). Pastthe focus, the pressure drops as the wavefront diverges (FIG. 13E). Inthe azimuth direction, the beam is relatively uniform (withoutapodization), with a variation of 2 dB relative to the maximum on theedges (FIGS. 136,13D). Beam steering is achieved for +/−18 degree angles(FIGS. 13F-13H) allowing for planewave compounding.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thescope of the invention. Accordingly, the invention is not limited exceptas by the appended claims.

I/We claim:
 1. A method of manufacturing a dual frequency transducercomprising: providing a high frequency (HF) transducer having a firstsheet of piezoelectric material including HF transducer elements, one ormore matching layers, and a HF frame; securing a surface of the sheet ofpiezoelectric material to a support frame, the support frame includingan epoxy material; forming at least one alignment post in the epoxymaterial; marking the at least one alignment post with one or morefiducials; positioning, on a first side of the HF transducer elements,an intermediate layer that is configured to absorb HF ultrasoundsignals, the intermediate layer including at least one slot configuredto engage with the at least one alignment post; aligning, with the oneor more fiducials, a low frequency (LF) transducer including: one ormore tabs with an alignment feature that aligns the LF transducer withthe one or more fiducials; a second sheet of piezoelectric materialincluding LF transducer elements; and one or more matching layers; andsecuring the one or more matching layers to the intermediate layer. 2.The method of claim 1, wherein the alignment feature comprises a hole onan alignment tab that is at a known position with respect to the LFtransducer elements of the LF transducer.
 3. The method of claim 1,wherein the intermediate layer is made of silicone powder-doped epoxy.4. The method of claim 3, wherein a surface of the support frame isfilled with an epoxy having a recess into which a portion of theintermediate layer is fitted.
 5. The method of claim 4, wherein theintermediate layer is adhered to the recess with the same epoxy in whichthe recess is formed.
 6. The method of claim 1, wherein the intermediatelayer is larger than an area of the HF transducer elements in thepiezoelectric layer of the HF transducer.
 7. The method of claim 1,wherein the intermediate layer includes at least one fiducial marked ata known position with respect to the location of the HF transducerelements, and wherein the alignment feature on the LF transducer isaligned with the at least one fiducial on the intermediate layer.
 8. Themethod of claim 1, wherein at least one of the tabs includes a hole toplace over the one or more fiducials on the HF transducer.
 9. The methodof claim 1, wherein the one or more alignment tabs in the LF transducerare incorporated into the one or more matching layers of the LFtransducer.
 10. The method of claim 1, wherein the first sheet ofpiezoelectric material further comprises an outer perimeter with anedge, and wherein the support frame surrounds the outer perimeter of thefirst sheet of piezoelectric material and is spaced from the edges ofthe outer perimeter such that kerf cuts that define individualtransducer elements in the sheet of piezoelectric material extend acrossa full width of the first sheet of piezoelectric material.
 11. A methodof manufacturing a dual frequency transducer comprising: providing ahigh frequency (HF) transducer comprising a sheet of piezoelectricmaterial including a number of HF transducer elements; providing abacking material that is constructed to absorb HF ultrasound signals andpass low frequency (LF) ultrasound signals; providing one or morefiducials marked on a first side of the HF transducer elements at aknown position with respect to the HF transducer elements; and providingan LF transducer including a number of LF transducer elements includingone or more alignment tabs that are positioned using the one or morefiducials to align the LF transducer elements with the HF transducerelements.
 12. The method of claim 11, further comprising incorporatingthe one or more alignment tabs in the LF transducer into a matchinglayer of the LF transducer.
 13. The method of claim 12, furthercomprising securing the one or more alignment tabs in an epoxy layerthat forms the matching layer.
 14. The method of claim 13, furthercomprising aligning the LF transducer elements with respect to the HFtransducer elements by placing a hole in the one or more alignment tabsover the one or more fiducials.
 15. A method of manufacturing a dualfrequency transducer comprising: providing a high frequency (HF)transducer including a first sheet of piezoelectric material including anumber of HF transducer elements, one or more matching layers, and asupport frame on a back side of the piezoelectric material configured tosupport one or more flex circuits with signal traces that connect to theHF transducer elements; providing one or more fiducials marked at knownpositions with respect to the HF transducer elements; providing anintermediate layer that is configured to absorb HF ultrasound signalsand positioned on a first side of the HF transducer elements; andproviding a low frequency (LF) transducer including a second sheet ofpiezoelectric material including a number of LF transducer elements, oneor more matching layers, and a flex circuit with signal traces thatconnect to the LF transducer elements, wherein the LF transducerincludes one or more tabs with an alignment feature that aligns the LFtransducer with a fiducial.
 16. The method of claim 15, wherein thealignment feature comprises a hole on an alignment tab that is at aknown position with respect to the LF transducer elements of the LFtransducer.
 17. The method of claim 15, wherein the intermediate layeris made of silicone powder-doped epoxy.
 18. The method of claim 17,wherein the support frame has a surface and wherein the surface of thesupport frame is filled with an epoxy having a recess into which aportion of the intermediate layer is fitted.
 19. The method of claim 18,wherein the intermediate layer is adhered to the recess with the sameepoxy in which the recess is formed.
 20. The method of claim 15, whereinthe first sheet of piezoelectric material further comprises an outerperimeter with an edge, and wherein the support frame surrounds theouter perimeter of the sheet of piezoelectric material and is spacedfrom the edges of the outer perimeter such that kerf cuts that defineindividual transducer elements in the sheet of piezoelectric materialextend across a full width of the sheet of piezoelectric material.